\(I\) : Following [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, we adopt the ResNet-152 [7]} to extract 2,048 \(7 \times 7\)  image feature maps which are further projected into 512 \(7 \times 7\)  feature maps, resulting \(I = \lbrace i_1,i_2,\dots ,i_{N_\text{I}}\rbrace  \in \mathbb {R}^{N_\text{I} \times d}\)  (\(N_\text{I} = 49\) , \(d = 512\) ).
